The present invention relates to an ultrasound imaging apparatus and a method thereof and, more particularly, to an ultrasound imaging apparatus and a method of the same for forming ultrasound images using a set of Golay codes having orthogonal property.
An ultrasound imaging apparatus transmits ultrasound signals to an object to be examined and processes signals reflected from the object to provide plane images of the object. It has been widely used in medical apparatuses.
As the power of the ultrasound used in ultrasound imaging apparatuses becomes strong, power of received ultrasound which is scattered or reflected from a medium becomes strong too thereby obtaining excellent signal-to-noise ratio (SNR). Accordingly, if possible, it is advantageous to use ultrasound having great amplitude, i.e., a transmitting wave of high voltage. Consequently, it is desirable to transmit ultrasound having great amplitude and short pulse length.
There is, however, certain limitation of using ultrasound of strong signal power in the application at human body since the ultrasound may influence on the body and also there are some limitations in the system""s hardware configuration. In order to resolve those limitations, it is suggested to use ultrasound signals of various code types. Ultrasound of longer length can be transmitted when the ultrasound signal of code type is used. Since the ultrasound of code type is used, the ultrasound of longer length is transmitted. Therefore, the power of instantaneous ultrasound is appropriately adjustable and also more energy is sent, thereby obtaining excellent SNR. Furthermore, received signals are compressed in their lengths by an appropriate signal process thereby obtaining enhanced resolution in the axial direction.
There are some kinds of codes roughly divided into a bi-phase code having 1 and xe2x88x921, and an arbitrary sequence code having arbitrary values. One can easily construct hardware of an ultrasound transmitter when he/she uses the bi-phase sequence code. Among those bi-phase sequence codes, the Golay code is known for realizing theologically ideal compression.
The Golay code has a set of complementary bi-phase sequences. Here, a predetermined bi-phase sequence set Ai having M number of sequences with length of L can be represented as follows:
Ai=[ai1,a2 . . . ,aiL]xe2x80x83xe2x80x83Eq. (1)
wherein i=1,2, . . . ,M, L is the length of the total sequences, and ai1,ai2, . . . ,aiL represent the biphase biphase sequences.
When the above sequence set satisfies the following equation Eq. (2), it is the complementary bi-phase sequence and the complementary bi-phase sequence set can be also used as the Golay code.                                           ∑                          i              =              1                        M                    ⁢                      xe2x80x83                    ⁢                                    ∑                              l                =                1                                            L                -                k                                      ⁢                          xe2x80x83                        ⁢                                          a                il                            ⁢                              a                                  i                  ,                                      l                    +                    k                                                  *                                                    =                  ML          ⁢                      xe2x80x83                    ⁢                      δ            ⁡                          (              k              )                                                          Eq.  (2)            
wherein k=0,1, . . . ,Lxe2x88x921, and xcex4(k) represents a general dirac function in which xcex4(k) is 1 in case of k=0 or xcex4(k) is 0 in case of kxe2x89xa00. Herein below, it will be explained about an ultrasound imaging apparatus using a general Golay code.
FIG. 1 is a block diagram of a conventional ultrasound imaging apparatus using Golay codes. As shown in the drawing, the conventional ultrasound imaging apparatus includes: an ultrasound transmitter 100; an transducer array 110; a transmitting/receiving switch 120; an analogue receiver 130; an A/D converter 140; a receiving beamformer 150; a pulse compressor 155; an echo processor 160; and a scan converter 170.
The ultrasound transmitter 100 applies voltage pulse into the transducer array 110 thereby outputting ultrasound signals from each transducer of the transducer array 110. In particular, each transducer generates ultrasound signals in reaction to the pulses applied from a pulser. While transmitting ultrasound signals, a timing point for generating ultrasound signals can be adjusted at each transducer of the transducer array so that the signals can be transmit-focused at a predetermined point in a region of interest. That is, the pulses are applied from the pulser with time delay into each transducer in order to make the signals to reach the predetermined point simultaneously thereby transmit-focusing at a desired position in the region of interest. As a method for deciding the pattern of transmission delay at each transducer, it has been used a fixed focusing technique which enables to bring a pulse energy of the ultrasound pulse into a predetermined point in a target object. In addition to the above, there has been recently proposed a synthetic aperture method as a suggestion to solve those limitations in resolution, which may be caused by using the fixed focusing technique.
The transmitting/receiving switch 120 acts for protecting the analogue receiver 130 from high voltage emitted from the ultrasound transmitter 100. In other words, the transmitting/receiving switch 120 switches properly the ultrasound transmitter 100 and the analogue receiver 130 while the transducer performs receiving and transmitting in turns.
The transducer array 110 has a plurality of transducers, for example 128 transducers, and each transducer reacts to a voltage applied from the ultrasound transmitter 100 and outputs ultrasound pulses. The fixed focusing technique or the synthetic aperture method as aforementioned can be used for such transmitting method. Herein, only some of the plurality of transducers are used for transmission of a time. In the fixed focusing technique, although an imaging apparatus includes 128 transducers, for example, only 64 transducers of them within a selected aperture transmit at one transmission of the ultrasound signals to a target object thereby forming one scan line.
The analogue receiver 130 receives reflected signals of ultrasound pulses returning from the object, in which the ultrasound pulses are outputted from each transducer of the transducer array 110; and also transmits processed signals into the A/D converter 140, in which the received reflected signals are amplified, removed the aliasing phenomenon and noise components, and attenuates equalization caused while the ultrasound passes through internal body. The A/D converter 140 converts an analogue signal from the analogue receiver 130 to a digital signal, and provides the digital signal to the receiving beamformer 150. The receiving beamformer 150 performs a dynamic receive-focusing by applying various amounts of delay, which vary with locations of the receive-focusing, to signals received from the A/D converter 140 and synthesizes the delayed signals.
The pulse compressor 155 processes the signals received from the receiving beamformer 150 in order to obtain resolution having similar quality as that of an ultrasound imaging apparatus of short pulse type. In the ultrasound imaging apparatus using long code like the Golay code, pulse compression is necessary because side-lobes of the received signals at the receiving beamformer 150 are too large to consist an image.
The echo processor 160 changes the pulse-compressed signals of the pulse compressor 155 into baseband signals, and extracts an envelope by using a quadrature demodulator, thereby obtaining data of a scan line.
The scan converter 170 stores the data obtained from the echo processor 160 in a memory (not shown), and matches a scan direction of the stored data to a pixel direction of a monitor. Meanwhile the data is mapped out at its corresponding pixel position on a monitor.
FIG. 2 illustrates an ultrasound transmitting process in the conventional ultrasound imaging apparatus as shown in FIG. 1. For convenience of explanation, the drawing only exemplifies a Golay code including a code sequence set of A1, A2 having length of L and M=2, and transmission by focusing at one focal point P.
In a first ultrasound transmission at one pulse repetition interval (PRI), all array elements 1axcx9c1h within a predetermined aperture of the transducer array 110 transmit ultrasound with increased amount of delay to an object so that the first code sequence A1 has the focal point P, and receive signals reflected from the object.
In a second ultrasound transmission at next PRI, all array elements 1axcx9c1h within the predetermined aperture of the transducer array 110 transmit ultrasound with increased amount of delay to an object so that the second code sequence A2 has the focal point P, and receive signals reflected from the object.
An image of the scan line can be displayed by using the signals received from those two transmissions. In particular, the signals received from the respective array elements laugh are pulse-compressed, and then selected amount of delay is loaded thereto, or alternatively the pulse-compression of the signals can be performed after obtaining the result of loading the selected amount of delay.
When the ultrasound is transmit-focused to a focal point with the use of a conventional bi-phase Golay code as described so far, the transmission must be performed as many times as the number of sequences included in one Golay code, i.e., M number of transmissions. Consequently, frame rate is reduced by 1/M compared with a general pulsing technique.
Therefore, an object of the present invention is to provide an ultrasound imaging apparatus and method that can provide excellent SNR and resolution in axial directions, which are advantages of the Golay code sets, by using the Golay code sets having orthogonal property, and also prevent reduction in the frame rate.
In accordance with one aspect of the present invention, there is provided an ultrasound imaging apparatus for forming an ultrasound image of an object, comprising: storing means for storing M number of orthogonal code sets having M number of complementary code sequences; transmitting means for sequentially transmitting M number of combined signals serving as an ultrasound transmission signals to M number of focal points within the object, in which M number of combined signals being obtained by combining the respective corresponding code sequences within the M number of code sets; receiving means for receiving signals reflected from the corresponding focal points to which the ultrasound signals are transmitted; and processing means for extracting data corresponding to the M number of code sequences within the stored M number of code sets from the reflected signals in order to form the ultrasound image of the object.
In accordance with another aspect of the present invention, there is provided an ultrasound imaging method for an object comprising the steps of: preparing M number of orthogonal code sets having M number of complementary code sequences; sequentially transmitting M number of combined signals serving as ultrasound transmission signals to M number of focal points within the object, in which the M number of combined signals are obtained by combining the respective corresponding code sequences within the M number of code sets; receiving signals reflected from the corresponding focal points to which the ultrasound signals are transmitted; and processing to form the ultrasound image of the object by extracting data for the M number of code sequences within the stored M number of code sets from the reflected signals.